require file in rails environment

There are different ways to load particular file in rails application environment if the required file exists.
1. Using RAILS_ROOT

if File.exists?(file=File.join(RAILS_ROOT,’config’, ‘initializers’, ‘smtp_gmail.rb’))

require file

end

2. Using Rails.root & join

require Rails.root.join(‘config’, ‘initializers’, ‘smtp_gmail.rb’)

3. Using Rails.root, join & exists? method.

if File.exists?(file = Rails.root.join(‘config’, ‘initializers’,’smtp_gmail.rb’))

require file

end

4. Using File and direct path to file

require  File.join(‘root’,’app’,’config’, ‘initializers’, ‘smtp_gmail.rb’)

Among all above methods of  loading file, Using Rails.root, join & exists? method seems to be pretty good to have.
Any improvements are most welcome!